1. Technical Field
The present invention relates to a position detection device, an optical device, a light source device, and a projector.
2. Related Art
In the related art, there are known light source devices that convert wavelengths of excited light emitted from solid-state light sources and emit the light as fluorescent light (for example, see JP-A-2012-137744).
A light source device disclosed in JP-A-2012-137744 includes a solid-state light source, a condensing lens, a plurality of lenses, a diffusion plate, and a phase difference plate (½ wavelength plate). Light emitted from the solid-state light source is incident on the phase difference plate via the condensing lens, the plurality of lenses, and the diffusion plate. The phase difference plate has a function of converting a polarization direction of incident linearly polarized light and is disposed in the light source device, for example, to be able to adjust rotation so that an S-polarized component of the light emitted from the phase difference plate is in the range of 10% to 30% and a P-polarized component is in the range of 90% to 70%.
Incidentally, the light source device disclosed in JP-A-2012-137744 includes an optical device that holds the phase difference plate described above to be able to adjust rotation. As such an optical device, for example, an optical device that includes a holder holding the phase difference plate and a gear transmitting power of a motor or the like to the holder is considered.
However, when the optical device including the gear is disposed in the light source device, dust such as grease attached to the gear or the like is considered to be attached to the phase difference plate (the optical element). Thus, there is a problem that the function of converting the polarization direction of the incident light deteriorates due to attachment of the dust to the phase difference plate.
In the optical device, a position detection device such as a photosensor accurately detecting a rotation amount of the gear is considered to be installed to change a rotation angle of the phase difference plate according to rotation amount of the gear. Such a position detection device includes, for example, a light emission unit and a light reception unit. A light-shielding portion moved according to a rotation amount of the gear is installed between the light emission unit and the light reception unit. By determining whether a light reception amount of light emitted from the light emission unit by the light-shielding portion in the light reception unit exceeds a predetermined threshold, a position of the gear is detected by the position detection device.
In the position detection device, however, the position of the gear is detected by determining whether the amount of received light in the light reception unit exceeds the predetermined threshold set in advance. Therefore, in a case in which the light emission unit and the light reception unit degrade, the light reception amount of light incident on the light reception unit does not exceed the predetermined threshold despite the fact that appropriately all of the light emitted from the light emission unit is incident on the light reception unit. In this case, there is a problem that the position of the gear or the like may be not accurately detected by the position detection device.